1. Field of the Invention
This invention relates to implantable medical devices adapted to deliver media for local and regional treatment.
2. Description of the State of the Art
This invention relates generally to implantable medical devices for treating bodily disorders. A typical treatment regimen with an implantable medical device involves implantation of a device at a selected treatment location. During treatment it may be necessary for the device to support body tissue. Therefore, the structure of a device may include load bearing structural elements or substrate to hold the device in place and to resist forces imposed by surrounding tissue.
The treatment of a bodily disorder may also involve local delivery of a bioactive agent or drug to treat a bodily disorder. The agent may be incorporated into the device in a variety of ways and delivered directly to an afflicted region at or adjacent to a region of implantation.
Additionally, in many treatment situations, the presence of the device is required only for a limited period of time. Therefore, a device may be composed in whole or in part of materials that degrade, erode, or disintegrate through exposure to conditions within the body until the treatment regimen is completed.
An example of such devices includes radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn which allows the stent to self-expand.
The stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength, which is the ability of a stent to resist radial compressive forces. Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. In addition, the stent must possess sufficient flexibility to allow for crimping, expansion, and cyclic loading.
The structure of a stent is typically composed of scaffolding or substrate that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed so that the stent can be radial'y compressed (to allow crimping) and radially expanded (to allow deployment).
Additionally, a drug-eluting stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolding may also serve as a carrier of an active agent or drug. Currently drugs or drug mixtures are typically released from coatings through diffusion or elution through coating. In addition, for pure drugs dispersed in coatings, the time frame of the therapeutic effect of the drug is relatively short. As a result, the treatment is limited to a region local to the region of implantation of the stent.
In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers can be configured to completely erode after the clinical need for them has ended.
In some treatment situations, local treatment of bodily tissue disorders with an implantable medical device may be difficult or insufficient. This insufficiency may be from the fact that tissue disorders may be diffuse and in multiple locations. Local treatment in such situations may require a multiplicity of devices. For example, vascular disorders can include lesions in multiple locations, such as diffuse lesions along vessels, multi-vessel lesions, and bifurcated vessel lesions. In addition, local treatment may be impossible because an afflicted region of tissue may be inaccessible to implantation of a device. For example, a diseased vessel may be too small for implantation of a stent. Thus, it would be desirable to have an implantable medical device that can be used to treat tissue disorders both local and regional to the location of implantation.